There is currently a demand for optical systems to afford ever increasing data transmission capacity. Two approaches are available to increase the transmission capacity of a single optical transmission path. One is to increase the bit rate; the other is to wavelength multiplex more channels on to the transmission path. Problems are associated with both approaches. Increasing the bit rate requires the use of higher speed, and hence generally costlier, electronic and opto-electronic devices, including optical modulators and photodetectors. Increasing the number of channels that are wavelength multiplexed on to a single transmission path increases the optical power density in that transmission path, and this in turn is liable to aggravate problems associated with non-linear effects and polarisation dependent effects in both the system fibre and in any rare-earth doped fibre amplifiers present in the transmission path.
One way of reducing these problems is to modulate two or more streams of pulses at a fraction of the required full bit rate of the or each wavelength channel, and then to recombine the streams in a bit interleaved fashion so as to achieve the full bit rate. Usually such signals are single mode. The combining of several single mode signals into a single combined single mode signal is not normally possible without engendering optical loss because this would involve infraction of the second law of thermodynamics. An exception to this generality occurs in the case of the combining of two polarised signals: two polarised signals can be combined, in theory without loss, if the signals are orthogonally polarised where they are being combined, the combining being effected using a polarisation beam splitter/combiner (PBS/C).
It is clear that, for such a system to work well, the time delay between the individual datastreams must be accurately controlled. Suppose the required precision is that the error shall not exceed xc2x14%. In the case of interleaving two 20 Gb/s bit streams to produce a single 40 Gb/s combined bit stream, the bit period of the 40 Gb/s bit stream is 25 ps, and hence the required timing precision is that the timing error between the two 20 Gb/s bit streams shall be to within xc2x11 ps. On the assumption that the light is propagating in single mode optical fibre waveguide having an effective refractive index of approximately 1.5, a differential delay of 25 ps is provided by two optical fibres differing in length by 5 mm with an error not exceeding xc2x1200 xcexcm. In any practical system of cleaving and splicing optical fibres, such precision in the location of fibre splices is not easily accomplished.
The present invention is directed to the pulse interleaving of m polarised optical bits streams of the same bit rate to produce an interleaved bit stream of m times that bit rate, and is particularly concerned with the provision of a convenient way of adjusting the relative timings so that the bits of the combined bit stream that are derived from said first bit stream are interleaved with the bits that are derived from said second bit stream with a predetermined relative timing relationship. When two bit streams are being interleaved in this way (i.e. m=2), typically, but not necessarily, the timing relationship is that providing substantially symmetrical interleaving of the bits derived from the two streams.
According to a first aspect of the present invention, there is provided a method of generating a polarisation diversity interleaved optical bit stream in which alternate bits have orthogonal polarisation states,
in which method first and second plane polarised optical carrier waves are fed in parallel to first and second optical modulators operating at the same bit rate to produce first and second optically plane polarised modulated bit streams,
wherein a polarisation beam-splitter/combiner is employed to combine said first and second modulated bit streams to form a combined bit stream with plane polarised bits, and
wherein the combined bit stream is launched into a length of birefringent optical fibre waveguide having principal polarisation planes which, at the launch of the combined bit stream into the fibre, are aligned with the polarisation planes of the combined bit stream, which length is such that, at the end remote from the polarisation beam-splitter/combiner, the bits of the combined bit stream that are derived from said first bit stream are interleaved with the bits that are derived from said second bit stream with a predetermined timing relationship.
The method may thus be seen to be capable of providing a relatively coarse measure of relative timing adjustment up until the PBS/C, and then, with the aid of a cut-back length of birefringent optical fibre waveguide, to refine this relative timing to a higher degree of precision.
A feature of this interleaving is that it produces a combined bit stream whose alternate bits are orthogonally polarised. This provides the additional advantage of reducing some adverse effects of optical non-linearities in the system fibre and polarisation hole burning in optical amplifiers.
According to a second aspect of the present invention, there is provided an optical bit interleaving assembly having,
a clock,
first and second state of polarisation (SOP) maintaining optical modulators, each provided with an optical input and an optical output,
an optical polarisation beam splitter/combiner (PBS/C) with first and second input ports and an output port, each of which ports has first and second principal polarisation planes (PPPs) with the property that a first polarised optical signal launched into the first input port with its SOP aligned with the first PPP of that first input port, and a second polarised optical signal launched into the second input port with its SOP aligned with the second PPP of that second input port, are both coupled substantially completely into the output port to emerge therefrom with their SOPs aligned respectively with the first and second PPPs of that output port,
first and second SOP-maintaining optical paths providing respective first and second optical couplings from the outputs of the first and second modulators to the first and second input ports of the PBS/C, and
a length of birefringent optical fibre waveguide having first and second PPPS,
wherein the clock is connected to provide a clock input to both of said first and second modulators,
wherein the first and second SOP-maintaining optical paths are optically coupled respectively with the outputs of the first and second modulators and the first and second input ports of the PBS/C such that any light emergent from the output of each of the first and second modulators with its SOP aligned with either one of the PPPs of that modulator is launched into the corresponding input port of the PBS/C with its SOP aligned with either one of the PPPs of that input port, and
wherein the length of birefringent optical fibre waveguide is optically coupled with the output port of the PBS/C with the PPPs of the fibre waveguide aligned with the PPPs of the output port of the PBS/C.
In the succeeding paragraph reference is made to xe2x80x98optical path lengthxe2x80x99. For the purposes of this specification, the optical path length of an optical element through which light propagates is defined to mean the product of the physical length through which that light propagates in the element with the effective refractive index experienced by that light in its propagation through the element.
According to a third aspect of the present invention, there is provided a method of making an optical bit interleaving assembly as set out in the preceding paragraph, in which method the first and second lengths of SOP-maintaining optical fibre waveguide are optically coupled respectively with the outputs of the first and second modulators before they are optically coupled with the first and second input ports of the PBS/C,
wherein, after the first and second lengths of SOP-maintaining optical fibre waveguide are optically coupled with the outputs of the first and second modulators, but before they are optically coupled with the first and second
input ports of the PBS/C, one of those lengths of SOP-maintaining optical fibre waveguide is trimmed in length to make the optical path length from its associated modulator to its end that is optically remote from the modulator differ, by a first predetermined amount with a first level of precision, from the corresponding optical path length between the other modulator and the remote end of its associated length of SOP-maintaining optical fibre waveguide, and
wherein, after the first and second lengths of SOP-maintaining optical fibre waveguide are optically coupled with the first and second input ports of the PBS/C, and also after the length of birefringent optical fibre waveguide is optically coupled with the output port of the PBS/C, the end of the length of birefringent optical fibre waveguide that is remote from said PBS/C is cut back in length until the difference in optical path length from each of the modulators to that remote end of the birefringent optical fibre waveguide reaches a second predetermined value with a second level of precision, which second level is proportionately more precise than the first level of precision.
Other features and advantages of the invention will be readily apparent from the following description of a preferred embodiment of the invention, from the drawings and from the claims.